The effect of in utero exposure to diethylstilbestrol (DES) on the immune system was investigated. Mice were exposed to DES through maternal dosing (0.1 mg/animal) on day 16 of gestation. Parameters of cell-mediated and humoral immune function were evaluated between 7 and 9 weeks of age. DES suppressed cell-mediated immune function in both males and particularly females. In contrast a sex interaction was noted in humoral immune function with enhanced response in males and either no effect or slight suppression in females.